Recently, along with the popularization of digital still cameras and camera-equipped cellular phones, low power consumption and miniaturization of solid-state imaging elements have proceeded and CMOS (Complementary Metal Oxide Semiconductor) image sensors have been more and more used other than conventional CCD (Charge Coupled Device) image sensors. These image sensors are each formed of a sensor section (imaging pixel section), in which a plurality of pixels are two-dimensionally arranged in a single semiconductor chip, and a peripheral circuit section arranged outside the sensor section.
As the structure to a CMOS image sensor, a “frontside-illuminated” structure and a “backside-illuminated” structure are known (see, for example, Patent Literatures 1 and 2). In the frontside-illuminated CMOS image sensor of Patent Literature 1, incident light from outside passes through a glass substrate and a cavity and enters each of microlenses. The light beams are converged by the microlenses, pass through a color filter layer and a wiring layer and enter photodiodes. The light incident on the photodiodes is photo-electrically converted to generate signal charges and then electrical signals are produced from signal charges to obtain image data.
In contrast, in the backside-illuminated CMOS image sensor of Patent Literature 2, photodiodes are formed on one of the surface of a semiconductor substrate. On the same surface, a color filter layer and microlenses are arranged. Above the microlenses, a glass substrate is arranged via an adhesive layer and a cavity. On the other surface of the semiconductor substrate, a wiring layer is provided. According to the backside-illuminated structure, since light incident on the microlenses is received by a light receiving section without passing through the wiring layer, attenuation of light by the wiring layer is avoided and the light-receiving sensitivity is enhanced.
As the backside-illuminated CMOS image sensor, a structure, which comprises an adhesive layer formed on a silicon substrate having microlenses and in the outer periphery of the microlenses so as not to cover the microlenses, a low refractive index layer formed in the cavity surrounded by the adhesive layer, and a glass substrate formed via the adhesive layer and the low refractive index layer, is disclosed (see Patent Literature 3).